Various systems have been developed for adjusting the positioning of an axle carried frame portion of a transport trailer of the type commonly used with a towing vehicle, such as a road tractor. There are also in use dolly devices which are towed by a first trailer and support the front end of a second trailer pulled in tandem in relation to the first trailer. Also certain of the above mentioned systems, which are adapted to adjust the position of the axle carried frame portion under a front trailer, are constructed so that the axle carried frame portion can be positioned for connection of a second trailer behind the front trailer.
Of prime importance, of course, in tractor-trailer operation, is that the entire rig, whether it is adapted to tow a second trailer, commonly called a "pup", or not, be entirely safe at highway speeds. Also of importance is that the trailer can be easily and quickly transformed from one mode to another by the driver of the tractor and that the maneuverability and general road performance is not significantly affected by such transformation.
Whether, the front trailer towed directly by the tractor has an auxiliary frame portion located to the rear of the normal frame portion of the trailer for the purpose of load distribution to the road or for the purpose of providing a connection to a second trailer, it is necessary, if the first trailer is of a type for unloading from the rear, that the auxiliary frame portion be readily detachable and again connectable from the first trailer to allow dumping of other types of unloading from the rear of the first trailer. Generally the connector systems of known types function in a manner which does not allow ready connection or disconnection or are not of a type which provide a connection of sufficient rigidity to allow the frame of the first trailer and that of an auxiliary frame to function as one continuous suspension frame.